Hard disk drives are mass storage devices that include a magnetic storage media, e.g. rotating disks or platters, a spindle motor, read/write heads, an actuator, a pre-amplifier, a read channel, a write channel, a servo circuit, and control circuitry to control the operation of hard disk drive and to properly interface the hard disk drive to a host system or bus. FIG. 1 shows an example of a prior art disk drive mass storage system 10. Disk drive system 10 interfaces with and exchanges data with a host 32 during read and write operations. Disk drive system 40 includes a number of rotating platters 12 mounted on a base 14. The platters 12 are used to store data that is represented as magnetic transitions on the magnetic platters, with each platter 12 coupleable to a head 16 which transfers data to and from a preamplifier 26. The preamp 26 is coupled to a synchronously sampled data (SSD) channel 28 comprising a read channel and a write channel, and a control circuit 30. SSD channel 28 and control circuit 30 are used to process data being read from and written to platters 12, and to control the various operations of disk drive mass storage system 10. Host 32 exchanges digital data with control circuit 30.
Data is stored and retrieved from each side of the magnetic platters 12 by the arm and interconnect 16 which comprise a read head 18 and a write head 20 at the tip thereof. The conventional read head 18 and write head 20 comprise magneto-resistive read head and thin-film inductive write head adapted to read or write data from/to platters 12 when current is passed through them. Arm and interconnect 16 are coupled to preamplifier 26 that serves as an interface between read/write heads 18/20 of disk/head assembly 10 and SSD channel 28. The preamp 26 provides amplification to the waveform data signals as needed for both read and write operations. A preamp 26 may comprise a single chip or may comprise separate components rather than residing on a single chip.
The magnetic flux transitions on the magnetic platter 42 are created by switching the write current polarity through the write head 20. The faster the write current switches polarity, the faster the change of the magnetic flux, and consequently more bits per inch can be stored in the media. To decrease the transition time of the media, an overshoot current is employed with the write driver signal.
To achieve the best high-speed performance in a preamplifier system, impedance matching techniques much be employed. One aspect of this is that the output impedance of the preamplifier write driver must match the interconnect which it drives. Currently, this may be done by customizing the write driver output impedance for each customer application since each application has its own unique interconnect characteristics. With an established write driver, this may be the only change between different applications. Furthermore, the interconnect may change during the lifetime of a customer application.
It would be very desirable and beneficial to have a write driver output impedance that can be programmable through the serial interface. This would save engineering work, reduce silicon tape-outs, mask costs, and increase chances of success in a customer application.